1. Was never serious about the job in the first place -
He's not serious about politics as evidenced by the poorly organized and unfocused campaign - and his alternately churlish-foolish behavior. Once admitted in an interview that entering the race was just done as a lark, and he never expected to succeed, even in the primaries. Michael Moore has a theory that problems with his Reality TV show are what prompted him to enter.
This by itself should be sufficient to disqualify, but there's also -
2. Reliance on fear-mongering as the primary campaign strategy -
Rage against the M&M's: Mexicans & Muslims are a serious growing threat. Muslims are a security threat, and Mexicans are all criminals. Crime on the streets of American cities is out of control. The 2nd amendment is in danger - Hillary and the liberals will "take your guns" - citizens will not be able to protect themselves against ever-increasing lawlessness. "Freedom" is slipping away.
3. Use of propaganda in place of facts and ideas -
Repetition of unsupportable statements: I will make America great again - I will build a wall along the Mexican border - If I'm elected, everyone will have a job. Exaggerating the number of people at speaking events. Takes quotes out of context to support his positions. Uses well-known people to endorse an idea or policy. Cynical identification with the working class by calling himself the "blue-collar billionaire".
4. Demagoguery -
Has established his base by appealing to the emotions and prejudices of a vulnerable segment in the voting population, namely the older, non-college-educated white males who would like see a return to the "way things used to be". Presents a distorted view of conservative principles, one that holds out the most extreme far-right beliefs as the only solution to overwhelming challenges the country now faces.
5. Anti-First Amendment -
Has Banned Washington Post and New York Times reporters from his public events. Is engaged in a campaign to vilify and intimidate the media, while at the same time using it to promote his propaganda.